Measuring devices or scanners of the type mentioned in the introduction are designed to measure a spatial region and/or an object three-dimensionally. Typical applications include, for example, the measurement of interiors, such as churches and factory buildings, large objects, such as buildings or aircraft, or else the forensic measurement of an accident site.
In order to measure these objects, by means of a laser scanner, a predefined spatial volume is scanned with a laser beam and the laser light reflected from the object is detected, wherein the angle information for the direction of the emitted laser beam and of the detected laser light is acquired for each point in time. By means of the detected laser light, the distance between a surface point situated in the spatial volume and the measuring device can be determined by triangulation and/or time-of-flight measurement or phase shift. Together with the angle information associated with said surface point, it is possible to calculate the spatial position of said surface point. From the sequence of measurement points recorded in this way, or the positions in space calculated therefrom, a three-dimensional model of the scanned surface, of the object or of the scanned environment is generated by corresponding software, e.g. in the form of a three-dimensional point cloud.
Devices for optically scanning an environment that are embodied as laser scanners, one of which in accordance with the prior art is illustrated by way of example in FIG. 1, usually comprise a measuring head 2, the housing 5 of which accommodates on one side a radiation source 6 for generating a transmission light beam 13 and a detector 8 for receiving the transmission light beam 13 reflected from objects in the environment, this being designated for short as reception light beam 17 or reflection radiation. In this case, “reflection radiation” is also understood to mean transmission light radiation scattered from objects in the environment in the direction of the measuring device, which occurs for example particularly in the case of objects having a very rough surface or else for example in the case of deciduous trees.
Furthermore, the housing 5 accommodates optical components 7 for collimating and diverting the transmission light beam 13 and the reflection radiation 17. The measuring head 2 or the housing is mounted on a base 3 rotatably about a base axis 4, said base generally having an adapter for fixing on a stand 19 or other carrier. The measuring head 2 or the housing 5 is generally rotatable about base axis 4 manually and/or in a motor-driven manner and in a manner supervised by a control unit 9. On an opposite side of the housing 5 relative to the beam source 6, the detector 8 and the optical components 7, a rotation unit 10 with a deflection element 22 is supported in the housing 5, by means of which rotation unit the transmission light beam 13 is emitted in a supervised manner, in an aligned manner into the environment, and the reflection radiation 17 is captured.
For this purpose, the rotation unit 10 with the deflection element 22 is mounted in the housing 5 rotatably about a rotation axis 11 in a manner driven by means of a motor 15. The rotation axis 11 is perpendicular to the base axis 4, and the deflection element 22 is generally arranged in a manner inclined by an inclination angle of 45° relative to the rotation axis 11. The point of intersection of base axis 4 and rotation axis 11 generally corresponds to the point of impingement of the concentrated transmission light beam 13 on the deflection element 22, said point also being designated as deflection point 23. By rotating the measuring head 2 about the base axis 4 and rotating the rotational unit 10 about the rotation axis 11, it is possible to carry out a three-dimensional scan.
On account of its complex internal optomechanical construction and the arrangement of the laser beam elements, the calibration of a scanner in accordance with the prior art is very demanding. The local, instrument-linked coordinate system of a scanner is described by parameters such as angle deviations and offsets of the rotary and targeting axes. Important calibration parameters include: tilting axis skew, the horizontal and vertical erroneous angles of the laser targeting direction, the angle error and the position of the deflection element or rotary mirror, the skew of the rotary axis (designated here generally as rotation axis) with respect to the vertical axis (designated here generally as base axis), etc. Hitherto, therefore, the determination of these parameters has been ascertained initially prior to delivery of the device by means of a factory calibration, which can be based on a two-position measurement, for example, as described in EP 2 523 017 A1.
A fast, efficient field calibration which would allow the user of the device to determine the present parameters on site has not been possible heretofore. Firstly, a fast efficient field calibration presupposes a precise setting unit, which is not available in every scanner, and, secondly, the instructions and mathematical models known from the literature for calibration with only one data set of a two-position measurement from a single instrument installation (rapidity and efficiency of the calibration!) are provided mainly for device constructions which do not correspond to the realized optomechanical construction of the scanner present, such that they are unusable for precisely determining the coordinate system of the given scanner. If the coordinates of reference points are not known beforehand and if only one data set of a two-position measurement from a single instrument installation is present, then the desired parameters cannot be determined by means of the models provided in the literature. This is because if no position information of the reference or target points is known, then the compensation problem in the evaluation of a data set from a two-position measurement becomes singular, and the calibration parameters are not fully determinable.
In order to be able to sight a target by means of a measuring device of the generic type, the prior art discloses various sighting units, such as, for example, a camera integrated into the housing of the measuring head on the side of the laser light source. Images acquired by said camera can be represented for sighting purposes in particular as a live image on a display. What is disadvantageous about measuring devices embodied in such a way is often the complexity of their operation. Moreover, the construction of such measuring devices, in particular for the optics used, is expensive. Moreover, the images necessarily recorded by a camera, which requires an additional power supply, have to be processed by corresponding software. Furthermore, a control and evaluation unit is necessary for the targeting process.
As an alternative form of providing the live image, the European Patent Application EP 12153163.6 in the name of the present applicant, and not yet previously published, discloses an eyepiece which is likewise arranged in the housing of the measuring head on the side of the camera and the laser light source and is equipped in particular with a target marking indicating the emission direction, e.g. in the form of a reticle. Said application also discloses an imaging system comprising an imaging optical unit and a display for graphically providing an imaging as a live image. The scanning region is selected according to a method corresponding to the prior art, involving the production of an overview scan and a corresponding overview image. With the aid of the image generated by the overview scan, the user then selects the region to be scanned, which is then scanned and measured in specific detail by means of a fine scan.